Prediction of Orthokeratology Lens Decentration with Corneal Elevation
Lens decentration is common and unavoidable to some extent during ortho-k treatment. By using a simplified method, we are able to predict the magnitude and direction of ortho-k lens decentration, which provides useful insights in screening for ideal ortho-k candidates and to make a quick decision when decentration happens.PURPOSE
The aim of this study was to investigate the influence of corneal elevation asymmetry on ortho-k lens decentration.METHODS
Thirty-six eyes of 36 subjects were fitted with four-curve reverse geometry ortho-k contact lenses. Corneal topography was collected before and 1 month after ortho-k lens wear. The difference in corneal elevation at the 8-mm chord of the respective two principal meridians of corneal astigmatism was calculated. Vector analyses were performed on these differences to calculate the magnitude and direction of a vector (corneal asymmetry vector). The relationship between the angle and magnitude of corneal asymmetry vector and lens decentration was analyzed.RESULTS
Baseline refractive sphere and cylinder for the 36 tested eyes were −2.84 ± 1.04 diopters (D) (range, −4.75 to −1.00 D) and −0.21 ± 0.28 D (range, −1.00 to 0 D), respectively. The mean magnitude of lens decentration was 0.72 ± 0.26 mm (0 to 1.34 mm). For overall displacement, inferotemporal decentration was the most common as observed in 24 eyes (67%). The mean angle of the corneal asymmetry vector (202 ± 39 degrees) was significantly correlated to the mean angle of lens decentration (200 ± 39 degrees) (r = 0.76, P < .001). The magnitude of corneal asymmetry vector significantly contributed to the magnitude of lens decentration (standardized β = 0.448, P = .002) whereas the other tested variables did not affect lens decentration (all P > .05).CONCLUSIONS
Lens decentration is a common phenomenon in ortho-k that mostly happens toward the inferotemporal quadrant of the cornea. The magnitude and direction of lens decentration are predetermined by paracentral corneal asymmetry.